These investigations are aimed at characterizing pituitary responses to GnRH (LHRH), and the modulation of these responses by sex steroids and other factors, including the pattern of GnRH administration (infusion and serial injections varying in amplitude and frequency), both in vivo and in vitro. Response patterns are evaluated by radioimmunoassay of LH in serially collected samples of blood or culture medium. In vivo studies utilize phenobarbital-treated cannulated rats, in which endogenous LH-releasing activity is minimal. In vitro studies are being done by perfusion of acrylamide gel-supported columns of trypsin-dispersed pituitary cells. The coordinate in vivo-in vitro approach is intended to define optimal conditions for examining morphological (immunohistochemical) and biochemical processes which are influenced by the GnRH stimulus and/or by feedback hormones, in order to define the modes of the regulatory hormone actions and interactions, as well as to provide the understanding of pituitary response capability needed to be able to base inferences about changes in the secretion of hypothalamic hormones on the changes in pituitary secretory activity which can be observed, in rats in which brain activity has not been impaired by phenobarbital treatment, in the course of various spontaneous and experimentally-induced endocrine events. Although current emphasis is on LH, the information gained from these experiments will be used in designing parallel studies on the control of FSH, TSH and prolactin secretion.